Will the sound quality deminish over the years?

Perhaps a another way to phrase the OP's Q is whether current model speakers use better materials and employ better technology that the SOTA of 40 years ago. And hence, sound better than the old stuff.

I suspect the answer is in the affirmative. Stated differently, if the OP was to stack an old pair of B&Ws against an equivalent current model, I surmise the newer model very well might sound better.

Technology has advanced across the board.

Short post-script to my last post.

If the OP's Q also goes to whether old speakers mechanically wear out like old car shocks over time, I surmise that some speaker parts will degrade over time. For example, the caps used in the cross-over may change values. Also, speaker surrounds may disintegrate over time. Possibly voice coil spiders too.

In short entropy is part of the natural world. Ain't no stopping it.

Zd542 - I didn't suggest that the change wasn't real. I did, however suggest that when his perspective changed he no longer viewed the Marantz as lacking highs, but rather have a more full mid and bass range. The sound of the Marantz was constant, but his assumption about it changed resulting in a different experience.

I just sold a 33 year old pair of Yamaha NS1000M speakers from my second
sound system. To my ears they still sounded just fine, even after 33 years
frequent use and replacement of two blown tweeter 10 years ago (don't
ask). Interestingly, my main speakers are B&W N801. The NS1000M, to my
ears, sounded very good in comparison to the N801; definitely not as broad
a frequency range, lucid or dynamic, but none the less, very good. The only
reason I sold the Yamaha's after 33 years was; the people who purchased
my home this year, specifically asked to purchase them, they liked the sound
and look that much! Interestingly, I sold them for more than I paid 33 years
ago.

I am sure your N802 speakers will sound just fine in 10-30 years time.

The OP inquired about loudspeaker degradation over time. Instead, he gets a discourse about tuners. Kijanki gave the only useful response.

Foam driver surrounds deteriorate and need replacing. How soon depends on the heat and humidity of the environment in which they operate.

I hope this helps.